Arterial angiography as commonly practiced involves an arterial injection of a contrast medium opaque to X-rays and subsequent X-ray photography of the artery section of interest distal to be injection. In contrast to tomography, angiography is practiced by obtaining images along a fixed direction; that is, the source of X-rays (or corresponding ultrasonic equipment) is fixed as is the location of the image receptor. Additionally, the patient is relatively immobile so that the artery section is maintained at a substantially fixed location and at a substantially fixed rotational disposition. A single picture or series of pictures of the artery section is taken as the contrast medium flows therethrough. A large amount of contrast medium is required in order to provide sufficient contrast to highlight the artery edge profile in each picture. Injection of this large amount of contrast medium poses a significant risk of trauma or fatality. As an example, coronary angiography has a fatality risk of the order of 0.01 to 0.001. On the other hand, if the contrast medium is injected in a vein, there is much lower risk of adverse consequences of the order of 0.0001. However, the resulting X-ray image has low contrast because of mixture, thereby dilution, of the contrast medium with substantial amounts of blood by the time it reaches the artery of interest. This low contrast makes it difficult to precisely define the artery edge, thereby making it difficult for a skilled person to determine whether corrective therapy is appropriate.
The present invention discloses a method and apparatus which eliminates the above-described disadvantages of current angiography, angiography being defined as any method for visualizing a blood vessel, the edge or lumen usually being of interest. Specifically, one aspect of the invention utilizes venous injection of the contrast medium while still providing an image which has a contrast equal to or better than that of an image obtained after direct arterial injection of the contrast medium. The method according to the invention includes the steps of injecting a contrast medium so that it will mix with blood to flow through a vessel or artery section of interest, obtaining multiple images of the vessel section as the blood/contrast medium mixture flows therethrough, and synthesizing a single composite image of the vessel section from information contained in the multiple images. In specific embodiments of the invention, a constrast medium opaque to X-rays is utilized. The vessel is irradiated by an x-ray source, the rays passing therethrough forming an image on an appropriately positioned X-ray sensitive film/screen combination. The synthesizing step includes registering the multiple images from landmarks that are contained within the patient and that have a relationship to the vessel of interest, and identifying density indices of corresponding segments of the registered images. A single image having corresponding indices either averaged or cumulatively added is formed from the identified density indices. In one embodiment, when the vessel does not move, the synthesizing step can include manually overlapping and registering transparent image negatives so that one may look through the registered negatives to observe the artery edge. This embodiment can be utilized for only a limited number of negatives because their cumulative opaqueness will soon mask the detail required for vessel resolution. In a preferred embodiment, computer technology is utilized whereby each image segment or element is assigned a digital representation according to its average density, a synthesized image being formed by the computer based upon the density information of each corresponding image segment.
One may utilize an X-ray sensitive phosphor in lieu of the X-ray sensitive film/screen combination, the phosphor being electronically scanned and the resultant signal being used to drive a video display tube or converted to a digital format for computer storage and subsequent processing.
If the vessel is moving during the time interval during which the images are obtained, a method of synthesizing a composite image includes the steps of defining anticipated image profiles based upon average vessel edge profiles of healthy vessels. The anticipated profile for the vessel at rest, called a base profile, is defined. An anticipated profile for each of the subsequent images is then derived with respect to the base profile. For example, an image taken 1/6th of a second after the image corresponding to the base profile would have a profile which represents the anticipated position of the vessel edge 1/6th of a second after that of the vessel edge corresponding to the base profile. Each image segment associated with the second image is then corrected by an amount equal to the offset between the base profile and the anticipated vessel edge profile for that picture, the correction being made so as to ascertain the position of the vessel edge at the time of the base profile position. This process is repeated for each of the subsequent images, thereby providing images in which all of the vessel edges are corrected so that they correspond to the time of the base profile. This offsetting can be most efficiently accomplished by a computer. Once the corrected images are developed, a synthesized image can be obtained as explained above.
Although in the preferred embodiment multiple images are obtained using X-ray photography, other techniques for obtaining vessel edge profiles could also be utilized. According to another aspect of the invention, the images could be obtained by the use of ultrasonic diagnostic instruments which are capable of distinguishing one type of tissue characteristic from another, thereby eliminating any need for injection of a contrast medium. When sound travels through soft tissue it suffers attenuation which increases with frequency. The greater the attenuation, the higher the relative reduction in the high frequency components. Means for developing a pictorial representation of the attenuation and reduction in high frequency components are well known in the art and can provide an alternative method of obtaining multiple images of the vessel, the images obtained being processed in the same manner as the X-ray images described above.